1. Field of the Invention
The present invention relates to a system and a method of desulfurizing a lean NOx trap (LNT). More particularly, the present invention relates to a system and a method of desulfurizing a lean NOx trap that improves fuel economy by not exiting from a desulfurization mode but performing desulfurization again under predetermined conditions even though a desulfurization stop condition of the LNT is satisfied.
2. Description of Related Art
Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and hydrocarbon (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reaction with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst is used as such a DeNOx catalyst. The LNT catalyst absorbs the NOx contained in the exhaust gas when air/fuel ratio is lean (smaller than 1), and releases the absorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich (greater than 1).
Since materials which absorb the nitrogen oxide in the LNT, however, are base materials, sulfur oxide (material obtained by oxidizing sulfur contained in fuel or engine oil) as well as the nitrogen oxide contained in the exhaust gas is absorbed. Sulfur poisoning in the LNT deteriorates purification efficiency of the LNT. Therefore, desulfurization of the LNT is necessary.
According to a conventional method of desulfurizing the LNT, the engine is controlled to enter a desulfurization mode so as to release the sulfur oxide poisoned in the LNT if an amount of the sulfur oxide poisoned in the LNT is greater than or equal to a predetermined amount. If it is difficult to continue the desulfurization because a driving condition is changed during performing the desulfurization, for example, the vehicle slows down or a road is congested, the desulfurization is stopped and the engine is returned to a normal driving mode. At this state, if a driving condition satisfies a condition where the desulfurization is possible, a temperature of the exhaust gas is raised and the engine enters the desulfurization mode again.
In addition, if the desulfurization mode is to be carried out just after the regeneration of the particulate filter is performed, the engine enters a regeneration mode of the particulate filter so as to enter the desulfurization mode. That is, after the particulate filter is regenerated, the engine enters the desulfurization mode. In further detail, if a desulfurization stop condition is satisfied, the engine is left from the desulfurization mode. After that, the engine does not enter the desulfurization mode immediately, but enters the regeneration mode of the particulate filter. After the particulate filter is regenerated, the engine enters the desulfurization mode again. Therefore, fuel consumption may be deteriorated, the engine may be damaged, and urea consumption may increase when regenerating the particulate filter.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.